Concrete Pipe — G-r-r-reat for Sanitary Sewer
Permarc Pipe Designers Conquer Concrete Pipe Corrosion
By Robin Wolf
In the home of Tony the Tiger, mascot for Kellogg’s Frosted Flakes cereal, concrete pipe is g-r-r-r-eat for a sanitary sewer receiving approximately 920,000 gal per day from the Kellogg cereal company, based in Battle Creek, Mich. The 3,000-ft run of sanitary sewer was needed by the City of Battle Creek to replace an 80-year-old clay pipeline that collected sewage for treatment in a facility located 25 miles away in Fort Custer, Mich. The primary challenge that had to be overcome by the pipeline designers was the acidic characteristic of the sewage. A low pH threatened the service life of the concrete pipeline and a sewage spill, unless the concrete mix was fortified with an admixture that would resist the potential effect of corrosion.
There are some 300 miles of sanitary sewer mains located throughout the Battle Creek metropolitan area. Wastewater generated by residences and businesses must be cleaned and treated to meet all federal and state standards before it is discharged into the Kalamazoo River. The city prefers specifying concrete pipe for sewers greater than 24 in. in diameter. The corrosive nature of the sewage, however, was the reason for first specifying the pipeline SDR 26 PVC, despite the city’s preference, and sections where the pipe would be buried deeper than 15 ft.
The specification was changed to reinforced concrete pipe once the client was assured that an admixture would prevent the corrosion. The additive ConShield prevents the colonization of aerobic (Thiobacillus) bacteria, which metabolizes hydrogen sulfide gas (H2S) and produces sulfuric acid on the walls of concrete pipe and manholes that, in turn, deteriorates concrete.
Anaerobic bacterias, which do not require oxygen for survival, form in raw sewage and produce hydrogen sulfide gas that has a distinct odor of rotten eggs. Factors that contribute to the rapid growth of anaerobic bacteria are temperature, retention time, high biochemical demand (BOD) and turbulence. Turbulence from force mains, drop structures in manholes, steep grade changes and pumping stations allows the hydrogen sulfide gas to release into the atmosphere in pipes and manholes. The gas is then converted into sulfuric acid (H2S04) by Thiobacillus bacteria that grows on surfaces of the concrete pipe and manholes above the wastewater flow. Only Thiobacillus can convert H2S to H2S04.
Pipe designers at Premarc are familiar with several additives used in concrete mixes to resist the corrosion of concrete by acidic soils and sanitary sewage. The design team proposed the use of ConShield, a liquid additive that molecularly bonds to the concrete mix to enhance the performance of the concrete sewer to meet the design life of the pipeline. The active ingredient in the admixture forms a colorless, odorless and positively charged polymer.
When Thiobacillus bacteria come in contact with the ConShield treated concrete, the cell membrane ruptures and the organism dies. Since nothing is transferred to the dead cell, ConShield treated concrete does not lose strength or weaken with continuous exposure to MIC corrosion.
The city approved the use of the ConShield additive in the cement mix designed for the concrete pipe and manholes and changed the specification. The challenge for the contractor, however, had just begun. The alignment of the new sewer was first planned within 10 ft of the existing clay pipeline. When the clay pipeline was uncovered, its integrity was breached and sections of pipe began to collapse when exposed.
The contractor — Peters Construction Co., a NUCA member based in Kalamazoo, Mich — was to install the new sewer within 10 ft of the existing clay pipeline. The contractor had to bypass the clay pipeline from manhole to manhole to avoid causing a sewage spill.
In addition, the Peters Construction Co. had a short time frame — between August 1 and Oct. 3, 2008 — to install the pipeline to meet Michigan DOT’s schedule for pavement replacement. Installation rates were improved utilizing concrete pipe vs. PVC pipe, since concrete pipe has narrower trench widths and is less dependent on the soil envelope to maintain its structure.
“We got high structural strength with protection from MIC in both the pipe and the manholes — at lower cost,” said Kurt Tribbett, City Engineer. “It was the logical decision.” Cost savings on the project were approximately $24,000.
During the planning and design stage of any sanitary sewer, the potential biochemical profiles of the system should be determined along with current rates of acid development, and projected rates for the design life of the sewer. Once determined, the pipe can be protected against the most common form of corrosion in municipal sewer systems with the simple addition of the ConShield additive.
Where acidic effluent is anticipated, designers need to determine the pH, including cyclic variations, as well as continuous or intermittent flow characteristics. The pH and total acidity for the design life of the system is critical to ensure the sewer meets its projected design life. In addition, designers must determine the potential for the development of sulfuric acid due to potential changes to the environment of the interior atmosphere of the sewer.
Technology is now in place for making concrete pipe more reliable than it has ever been before. Decades of research and development of many aspects of concrete pipe have enabled concrete pipe producers to change concrete mixes and pipe design to provide products that can withstand a complete range of underground environments and effluent profiles.
The Battle Creek concrete sanitary sewer that receives considerable effluent from the Kellogg Co. was carefully designed with full knowledge of the nature of the sewage and design life of the pipeline. An innovative additive used to create an environment that can accommodate reinforced concrete pipe has also contributed to the city’s sustainable infrastructure.
Robin Wolf is the Director of Marketing at Premarc Corp., based in Durand, Mich. Robin can be reached at (616) 437-0781.
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